1. Field of the Invention
This invention relates to constructions of floating vessels and to arrangements for effecting stabilization and steadiness of such vessels.
2. Description of the Prior Art
U.S. Pat. Nos. 3,271,964, 3,490,406, 3,610,193, 3,673,974 and 3,830,176 all disclose semi-submersible vessels or offshore rigs.
Semi-submersibles, monohull vessels and, in some cases, jack-up type rigs provide alternatives to conventional fixed jackets for supporting process facilities. However, the deck space, payload and storage capacities required cannot always be met by existing semi-submersibles or jack-up designs and in some field locations wave induced motions can impede continuous process operations on some monohull process and storage vessels.
Fundamental requirements for floating vessels indicated the need for the following characteristics:
The wide variety of processing facilities can require large deck spaces and high payload capacities.
A floating process vessel's motions response to waves must be sufficiently small that down time of motion sensitive process equipment is kept to an insignificant level.
Both semi-submersibles and monohull surface vessels have shortcomings when viewed against the above criteria. Semi-submersibles have low wave induced motion characteristics but suffer from low payload capacity. Monohull vessels, on the other hand, offer high payload capacity but have higher wave induced motions than semi-submersibles.
The low wave induced motions of semi-submersibles are due to the open "space frame" pontoon/column configuration which allows inertia and pressure induced wave forces to partially cancel each other, thereby reducing the level of wave forces on the vessel. The low waterplane area of a semi-submersible also gives rise to high natural periods which are helpful to vessel motions at predominantly occurring wave periods. In contrast a monohull vessel achieves its high load carrying capacity due to its large waterplane area but this contributes to its relatively higher wave induced motions.
In floating vessels, stability and wave included motions impose conflicting demands on the vessel design. Adequate stability requires high hydrostatic "stiffness" (in roll or pitch) of the vessel whereas low wave induced motions are obtained by keeping the hydrostatic stiffness to low values.
It is an object of the present invention to devise a vessel having lower motion response to waves at predominantly occurring wave periods as compared with a typical equivalent monohull vessel while providing a higher payload capacity than an equivalent semi-submersible.